1. Technical Field
The present invention relates to performance of initial ranging in a cognitive radio network, and more particularly, to a method and a system for performing initial ranging in a cognitive radio network capable of minimizing calculation in a base station in consideration of an operation environment of a cognitive radio based network, for example, a wireless regional area network (WRAN) system and estimating an approximate time offset.
2. Description of the Related Art
IEEE802.22 is a standard defining a physical (PHY) layer and a data link (MAC) layer of a wireless regional area network (WRAN) system providing a wireless communication services in a cell coverage from 30 km to 100 km using a television (TV) idle channel.
A new customer-premises equipment (CPE) should perform an initial access procedure in order to access a WRAN network, and a ranging process of performing time synchronization so that signals transmitted from CPEs may arrive at a base station at a predetermined point in time should be first performed before a meaningful message is transmitted.
This procedure is called initial ranging, the CPE transmits a randomly selected code, and the base station detects the corresponding code and controls the corresponding CPE to perform the initial access process.
In the IEEE802.22 standard, five orthogonal frequency division multiplexing (OFDM) symbol sections are defined as sections for the initial ranging in consideration of a cell coverage of 100 km, and each CPE selects a random code for the initial ranging and generates and transmits three phase-continuous OFDM symbols.
The base station should detect the transmitted code. In addition, the base station estimates an amount of phase offset depending on an arrival delay together with the selected code to inform the CPE transmitting the ranging signal of the time offset, and the corresponding CPE reflects the time offset and again transmits the ranging signal.
When an accurate point in time is confirmed, the CPE transmits a message to the base station, thereby making it possible to perform the initial access procedure.
An initial ranging signal used in the WRAN system is generated depending on the following Equation 1 so that phase discontinuity between the OFDM symbols is not present.
                              s          ⁡                      (                          n              ,              l                        )                          =                                            ∑                              k                =                0                                                              N                  FFT                                -                1                                      ⁢                                          [                                                      b                    k                                    ·                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                        k                          ·                          l                          ·                                                      N                            CP                                                                                                    N                          FFT                                                                                                                    ]                            ·                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                            k                      ·                      n                                                              N                      FFT                                                                                                    =                                    ∑                              k                =                0                                            N                                  FFT                                      -                    1                                                                        ⁢                                          b                k                            ·                              ⅇ                                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                  k                        ·                                                  (                                                      n                            +                                                          l                              ·                                                              N                                CP                                                                                                              )                                                                                            N                        FFT                                                                              ,                                                                                        [                  Equation          ⁢                                          ⁢          1                ]                                          b          k                =                  {                                                                                          2                    ·                                          (                                                                        1                          2                                                -                                                  C                          i                                                                    )                                                        ,                                                                                                  i                    =                                          i                      +                      1                                                        ,                                      k                    ∈                    R                                                                                                                        0                  ,                                                                              k                  ∉                  R                                                                                                                where                                                                                          s              ⁢                              (                                  n                  ,                  l                                )                            ⁢                                                          ⁢              is              ⁢                                                          ⁢              the              ⁢                                                          ⁢                              l                th                            ⁢                                                          ⁢              OFDM              ⁢                                                          ⁢              symbol              ⁢                                                          ⁢              for              ⁢                                                          ⁢              initial              ⁢                                                          ⁢              ranging              ⁢                                                          ⁢              with              ⁢                                                          ⁢              the                                                                          sample              ⁢                                                          ⁢              index              ⁢                                                          ⁢                              n                .                                                                  ⁢                l                            ⁢                                                          ⁢                                                is                  ⁢                                                                          [                                      0                    ,                    1                    ,                    2                                    ]                                .                                                                                        k              ⁢                                                          ⁢              is              ⁢                                                          ⁢              the              ⁢                                                          ⁢              subcarrier              ⁢                                                          ⁢              index              ⁢                                                          ⁢              in              ⁢                                                          ⁢              the              ⁢                                                          ⁢              channel                                                                                          C                i                            ⁢                                                          ⁢              is              ⁢                                                          ⁢              the              ⁢                                                          ⁢              ranging              ⁢                                                          ⁢              code              ⁢                                                          ⁢              defined              ⁢                                                          ⁢              in              ⁢                                                          ⁢              9.9              ⁢              .3              ⁢              .1              ⁢                              .1                .                                                                                        i              =                                                [                                      0                    ∼                                                                  N                        code                                            -                      1                                                        ]                                ⁢                                                                  ⁢                where                ⁢                                                                  ⁢                                  N                  code                                ⁢                                                                  ⁢                is                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                length                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                CDMA                                                                          code                                                              R              ⁢                                                          ⁢              is              ⁢                                                          ⁢              the              ⁢                                                          ⁢              set              ⁢                                                          ⁢              of              ⁢                                                          ⁢              index              ⁢                                                          ⁢              of              ⁢                                                          ⁢              subcarriers              ⁢                                                          ⁢              within              ⁢                                                          ⁢              the              ⁢                                                          ⁢              ranging                                                            subchannel                                                                                                N                  FFT                                ⁢                                                                  ⁢                is                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                size                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                K                ⁢                                                                  ⁢                FFT                            ,              2048                                                                                          N                CP                            ⁢                                                          ⁢              is              ⁢                                                          ⁢              the              ⁢                                                          ⁢              size              ⁢                                                          ⁢              of              ⁢                                                          ⁢              the              ⁢                                                          ⁢              cyclic              ⁢                                                          ⁢              prefix                                                                      
Since a phase among the three OFDM symbols is continuous, in the case in which fast Fourier transform (FFT) is performed except for both distal ends of the ranging signal, an inter-cell interference (ICI) phenomenon depending on the phase discontinuity does not appear. Each CPE transmits the initial ranging signal in a region for performing ranging among upstream resources shown in FIG. 1, and a time error appears from a first symbol start position of a resource allocated for the ranging depending on a round-trip delay corresponding to a distance difference between the base station and the CPE.
Therefore, the ranging signal is positioned in first to third symbol sections in the case in which a delay is not substantially generated and is positioned in third to fifth symbol sections in the case in which a maximum delay is generated. In this case, since a delay offset (phase offset) that is to be estimated has a range corresponding to the number of 2048 samples, which is an FFT size, an integer multiple error of the OFDM symbol may not be detected.
Therefore, a process of detecting approximate positions of the symbols is required. In addition, since all selectable random codes should be multiplied by codes corresponding to combinations of phase offsets for 2048 sample delays of the respective codes in a method of detecting a signal, a number of calculation processes are required.